go to the my-europe-visa website; all requirements are very clearly spelled out.http://www.my-europe-visa.com/
i recommend the online application form; it does speed things up (even though you take them the printed copy)

need either proof of booked accommodation or a letter from a resident saying you will stay with them
i simply made a bunch of hotel bookings on booking.com; printed the booking confirmations as part of my visa submission and cancelled all the bookings after i got the visa.
NB!: make sure that bookings at a hotel can be cancelled for free before you make the booking! some hotels require a deposit & have no-cancellation clauses; most are free cancellation though. the website makes it very clear what the terms of each hotel's bookings.

enjoy!
ps: welcome to pm me if you have more questions
psps: i got my visa via the italian consulate; but as far as i know requirements are the same (?)

Also works to just submit an explanatory letter with your visa application. Explain the purpose of your visit, ie. you are going rock climbing, and that you will be camping close to the crags. Provide as much detail as possible, specifically your itinerary and the names and contact numbers of the campsites. Have never been refused a visa following this approach.

1. The plan is to fly into Marseilles but out of Barcelona. Any idea if this will this make the visa application more likely to fail? If so then I will rather arrive and depart Marseilles.

2. Anyone have any opinion on which Willem's approach is best? That is, is it best to (a) book hotels then cancel, or to (b) own up to being a camper during the visa interview? (Despite neither approach meeting the official requirement of all accommodation prepaid.)

Any comments appreciated.

(I am sure there is a special place in hell reserved for the bureaucrats who formulate these impossible rules. )

the my-europe-visa website specifies the following wrt accommodation: "Proof of accommodation: Confirmed hotel booking or original certificate of board and lodging ("attestation d'accueil")."
and for the online application form;
"Enter the name of the person inviting you , or the contact details of your hotel. Please follow this format: Name (Host/Hotel), Address (with Zip code and City), and Telephone Number(s) . If you are staying in several hotels, use the details for the first night in France."

I could not find any reference to pre-paid accommodation?

Anyways, the visa process is not to keep good, money spending tourists out of the country - I expect them to be sensible about it.

ps: i made bookings at different hostels / hotels for about a week at a time (5 or 6 hotels) to seem authentic